A simple level of ab initio molecular orbital theory with a split-valence shell basis with d-type polarization functions (6–31G*) is used to predict equilibrium geometries for the ground and some low-lying excited states of AHn molecules and cations where A is carbon, nitrogen, oxygen or fluorine. The results are shown to be close to the limit for single determinant wave functions in cases where corresponding computations with more extensive bases are available. Comparison with experimental results also shows good agreement although a systematic underestimation of bond lengths up to 3 per cent is evident. For systems where no experimental data are available, the results provide predictions of equilibrium geometry.

Accuracy of AH n equilibrium geometries by single determinant molecular orbital theory

Mise au point. Theorie des effets de couplage vibronique multimodes. Probleme a 2 etats. Couplage vibronique mettant en jeu des modes et des etats degeneres. Effets du couplage vibronique multimodes en spectroscopie. Comportement statistique des niveaux d'energie vibroniques. Intersections coniques et evolution temporelle de la fluorescence

Multimode molecular dynamics beyond the Born-Oppenheimer approximation

It is shown that the convergence of the configuration expansion of a many‐electron wavefunction may be drastically improved without significantly complicating the energy matrix elements by using partially non‐orthogonal orbitals. The wavefunction thus obtained is the many‐electron analog to the natural orbital expansion of a two‐electron wavefunction. The orbitals involved may be approximated by pseudonatural orbitals (``PNO–CI''), and an efficient way of calculating these orbitals is presented. An approximate treatment of multiple substitutions is discussed yielding a coupled electron pair theory formulated within the CI scheme. These methods are applied to the ground state and three ionized states of methane. Including only double substitutions, the computed upper bound to the energy of the ground state is −40.4584 hartree, i.e. 0.057 above the experimental nonrelativistic energy. About 83% of the total correlation energy is accounted for variationally, whereas the coupled‐pair approximation yields 89%....

PNO–CI Studies of electron correlation effects. I. Configuration expansion by means of nonorthogonal orbitals, and application to the ground state and ionized states of methane

Kraitchman has shown that a single isotopic substitution on an atom is sufficient to determine directly the coordinates of that atom with respect to the principal axes of the original molecule. Kraitchman's formulas represent exact solutions of the equations for the equilibrium moments of inertia. However, the effects of the zero‐point vibrations are such that the coordinates obtained by substitution from the ground state moments of inertia I0 are systematically less than r0. These coordinates have here been called r (substitution) or rs, and it is found that rs≃(r0+re)/2, and Is= ∑ imirsi2≃(I0+Ie)/2.In the usual method of solution, the coordinate of one atom is determined from the equation for I0, and therefore the difference I0—Is must be made up by this one coordinate. This introduces a large error in the structures normally determined from ground state constants, and results in variations of 0.01 A in structures determined from different sets of isotopic species. If instead, we obtain the structure on...

A new criterion for the determination of molecular structures from ground state rotational constants

Multimode Molecular Dynamics Beyond the Born‐Oppenheimer Approximation

Molecular orbital theory of the electronic structure of organic compounds. X. Systematic study of geometries and energies of AHn molecules and cations

Ab initio molecular orbital studies on C2H6 + support an assignment of 2 A 1g (D 3d point group symmetry) for the ground state of this cation. This differs from the prediction of Koopmans' theorem which suggests a 2 Eg state for vertical ionization. Approximate force constants and vibrational frequencies are calculated for the ethane cation in the 2 A 1g state and are found to be consistent with the vibrational progression observed in the experimental photoelectron spectrum.

Theory of the ionization of ethane

Molecular Orbital Structures for Small Organic Molecules and Cations

Mo-theorie der elektronenstruktur organischer verbindungen 10. mitt. systematische untersuchung der geometrien und energien von ah(n)-molekuelen und -kationen

L. A. Curtiss

Papers

Molecular orbital theory of the electronic structure of organic compounds. X. Systematic study of geometries and energies of AHn molecules and cations

Theory of the ionization of ethane

Molecular Orbital Structures for Small Organic Molecules and Cations

Mo-theorie der elektronenstruktur organischer verbindungen 10. mitt. systematische untersuchung der geometrien und energien von ah(n)-molekuelen und -kationen